If you got that, you could be mastering your stereo mixdown files even using the default GarageBand instalation – although it may get a bit awkward at times. :)

Other than that, here is probably the best place to start:
http://productionadvice.co.uk
(I'm not affiliated with that site, but the owner has done a couple of great mastering jobs for me in the past, and his online articles and tutorials are very helpful.)